1. Field of the Invention
The present invention relates to an extreme ultraviolet light source apparatus generating plasma by irradiating a target with a laser light and outputting ultraviolet light emitted from the plasma.
2. Description of the Related Art
In recent years, along with a progress in miniaturization of semiconductor device, miniaturization of transcription pattern used in photolithography in a semiconductor process has developed rapidly. In the next generation, microfabrication to the extent of 70 nm to 45 nm, or even to the extent of 32 nm and beyond will be required. Therefore, in order to comply with the demand of microfabrication to the extent of 32 nm and beyond, development of such exposure apparatus combining an extreme ultraviolet (EUV) light source for a wavelength of about 13 nm and a reflection-type reduction projection optical system is expected.
As the EUV light source, there are three possible types, which are a laser produced plasma (LPP) light source using plasma generated by irradiating a target with a laser beam, a discharge produced plasma (DPP) light source using plasma generated by electrical discharge, and a synchrotron radiation (SR) light source using orbital radiant light. Among these light sources, the LPP light source has such advantages that luminance can be made extremely high as close to the black-body radiation because plasma density can be made higher. Moreover, the LPP light source also has an advantage that luminescence only with a desired wavelength band is possible by selecting a target material. Furthermore, the LPP light source has such advantages that there is no construction such as electrode around a light source because the light source is a point light source with nearly isotropic angular distributions, extremely wide collecting solid angle can be acquired, and so on. Accordingly, the LPP light source having such advantages is expected as a light source for EUV lithography which requires more than several dozen to several hundred watt power.
In the EUV light source apparatus with the LPP system, firstly, a target material supplied inside a vacuum chamber is excited by being irradiated with a laser light and thus be ionized to become plasma. Then, a cocktail light with various wavelength components including an EUV light is emitted from the generated plasma. Then, the EUV light source apparatus focuses the EUV light by reflecting the EUV light using an EUV collector mirror which selectively reflects an EUV light with a desired wavelength, e.g. a 13.5 nm wavelength component. The reflected EUV light is inputted to an exposure apparatus. On a reflective surface of the EUV collector mirror, a multilayer coating with a structure in that thin coating of molybdenum (Mo) and thin coating of silicon (Si) are alternately stacked, for instance, is formed. The multilayer coating exhibits a high reflectance ratio (of about 60% to 70%) with respect to the EUV light with a 13.5 nm wavelength.
Here, as mentioned above, plasma is generated by irradiating a target with a laser light, and at the same time, particles (debris) such as gaseous ion particles and neutral particles, and tiny particles (metal cluster) which have not been able to become plasma fly out around thereof from a plasma luminescence point. The debris fly toward surfaces of various optical elements such as an EUV collector mirror located in the vacuum chamber, focusing mirrors or focusing lenses for focusing a laser light on a target, and other optical system for measuring an EUV light intensity, and so forth. Therefore, fast ion debris with comparatively high energy erode surfaces of optical elements and damage reflective coating and non-reflective coating of the surfaces. As a result, the surfaces of the optical elements will become a metal component, which is a target material. On the other hand, slow ion debris with comparatively low energy and neutral particle debris will deposit on surfaces of optical elements. As a result, a layer of a compound of metal, which is a target material, is formed on the surfaces of the optical elements. As a result of the debris entering as mentioned above, the reflective coating and the non-reflective coating of each optical element is damaged or a compound layer is formed on the surfaces of the optical elements, whereby reflectance or transmittance of the optical elements decrease and the optical elements become unusable.
In this respect, Japanese patent application Laid-Open No. 2005-197456 discloses a technique such that debris flying from plasma is trapped by a magnetic field generated inside an optical collecting system by a magnetic field generator when current is supplied to the magnetic field generator. According to this technique, by locating a luminescence point of an EUV light within the magnetic field, ion debris flying from the plasma generated around the luminescence point converge in a direction of the magnetic field by Lorentz force by the magnetic field. As a result, contamination of neighboring optical elements with debris and damages of the optical elements can be reduced.
On the other hand, US patent application Laid-Open No. 2008/0197297 discloses a technique with which generated debris are trapped around a plasma luminescence point by a magnetic field generated as a result of making the plasma luminescence point surrounded with wiring and passing current to the wiring.